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U. S. DEPARTMENT OF AGRICULTURE, 

BUREAU OF ENTOMOLOGY CIRCULAR No. 164. 

L. O. HOWARD. Entomologist *nd Chief oi Bureau. 



THE GIPSY MOTH AS A FOREST INSECT. 

WITH M SeESTIOHS kS TO lis CONTEOL. 



BY 



W. F. FISKE, 



673W-13 1 



WA4HINCTON : GOVERNMENT WIINTINO OFFKE : 1(11 




BUREA U OF EKTOMDWa Y. 

L. 0. Howard, Entomologist and Chief of Bureau. 
C. L. Marlatt, Entomologist and Acting Chief in Absence of Chief. 
R. S. Clifton, Executive Assistant. 
W. F. Tastet, Chief Cleric. 
F. II. Chittenden, in charge of truck crop and stored product insect investigations. 
A. I). Hopkins, in charge of forest insect investigations. 
W. I). Hunter, in charge of Southern field crop insect investigations. 
F. M. Webster, in charge of cereal and forage insect investigations. 
A. L. Quaintance, in charge of deciduous fruit insect investigations. 
E. V. Phillips in charge of ha culture. 
Rolla P. Currie, in charge of editorial work. 
Mabel Colcord, in charge of library. 

Preventing Spread of Moths. 

laboratory. 

(At Melrose Highlands, Mass.) 

A. F. Burgess, in charge of biological i /instigations: 

W. F. Fiske, in charge of parasite and disease investigations. 

Kenneth W. Brown, C. W. Collins, J. J. Culver, John E. Dudley, Jr., 
Hartley R. Gooch, Chas. W. Minott, F. H. Mosher, Harold A. Preston, 
E. A. Proctor, John V. Schaffner, Jr., M. B. Shepherd, C. W. Stock well, 
J. N. Summers, W. B. Turner, Reginald Wooldridge, assistants. 

field work. 

D. M. Rogers, in charge of Eastern territory. 
L. II. Worthley, in charge of Western territory. 

Harold A. Ames, I. L. Bailey, Henry X. Bean. Frank W. Graves, Jr., II. L. 
McIntyre, D. G. Murphy, Charles E. Totman, H. W. Vinton, assistants. 
ii 



Circular No. 164 

I nited States Department of Agriculture, 

BUREAU OK ENTOMOLOGY. 
L. O. HOWARD, Entomologist and Chief of Bureau. 



Till QIPST MMTII AS v FOHKsl INSECT, WITH SUGGESTIONS kS 

TO ITS CONTROL 

By W. I'. FisKi 
Agi 

mi OIFST-MOTB Mil \ii<>\. PAST \\i> PRESENT. 

It has been said of the gipsy moth thai the caterpillar is almosl 
omnivorous so far as foliage is concerned, and the earrj reports ] >u I >- 
lished by the State Board <>l' Agriculture of Massachusetts abound in 
references confirmatory of this statement. It is in facl incontro- 
\ ertible, from the ma— of evidence furnished by these reports as well 
as by the contemporaneous accounts in the press, thai thegipsj moth 
was formerly almosl unique amongst injurious insects in its ability 
to destroy all sorts of vegetation. Upon the occasion of it- historic 
outbreak in Medford and Maiden, beginning about 1889, mid again in 
the larger outbreak following a few years after the extermination 
work was concluded in 1900, not only forest, -hade, and ornamental 

t fees hut Orchards, gardens, and Held- were defoliated and deva-t at ed. 

Ami when the food supplj was exhausted the starving caterpillars, by 
force of numbers alone, constituted a veritable plague, rendering the 
streets almost impassable to pedestrians, massing upon and entering 
houses, and infesting the bedrooms, the kitchen-, and even the dining 
tables as well as all outdoors. 

It i- needless to state that these conditions no longerprevail. ( later- 
pillars there are, during their season ; egg masses in varying abundance 
are everywhere to be found in neglected woodland-, and thousands 
of dead and dying tree- -tand as evidence that unless it he rendered 
-till further innocuous the gipsy moth i- -till a very living factor to be 
considered in the future of American forestry. Bui the accounts of 
its earlier depredations seem all hut incredible when compared with 
condition- to-dav. It i- no longer prominent as a field and garden pesl 

nstderattao < noth (Porthr/ria dhpar I It" disease, 

natural raa ol trees to attack by th<? gipsy moth, as applied bribe manajrenv 

knots. 

1 



2 THE GIPSY MOTH AS A FOREST INSECT. 

As an orchard insect it is not infrequently eclipsed by the American 
tent caterpillar. Even the forests have suffered less than early pre- 
dictions would have led one to expect. It is certain that the situation 
has become measurably improved within recent ""ears. 

CAUSES OF THE IMPROVED CONDITIONS. 

This obvious improvement is in part only apparent and in part 
very real and due to a variety of causes. The apparent amelioration 
is due to the fact that the gipsy moth is at present most active in a 
, belt of towns beyond the limits of the densely populated metropolitan 
area. Were Boston's parks again to be infested as formerly; were the 
forests in the Middlesex Fells, for example, to be defoliated, a wave 
of remonstrance would arise which might be heard halfway across 
the continent. But a thousand acres of forest in the sparsely popu- 
lated and financially poor towns 30 to 50 miles away may be repeatedly 
defoliated and ultimately destroyed without creating more than a 
ripple in comparison. And this latter is precisely what is taking 
place at the present time. It will be necessary to wait until in its 
slow progress the gipsy moth invades another great metropolitan 
area before popular interest will be aroused to an extent comparable 
to that existing in Massachusetts a few years ago. 

The real amelioration so noticeable in the metropolitan district, and 
distinctly in evidence everywhere, is due to at least four main causes: 
(1) The perfection and standardization of the methods for artificial 
repression; (2) the death of a large proportion of the more susceptible 
trees or their removal from the infested woodlands; (3) the importa- 
tion of parasitic and predatory insect enemies; and (4) the develop- 
ment of the "wilt" disease. 

As it is intended at this time to consider the gipsy moth strictly as 
a forest insect no mention need be made of the methods for artificially 
suppressing it. On account of their expense these methods can not 
be used in forests other than those which it is desired to protect for 
aesthetic and sentimental reasons. 

RESULTS OF PARASITE IMPORTATION. 

There are about 30 species of insect enemies of the gipsy moth which 
appear to be of importance in checking its increase in Europe and 
Japan. All of the promising species have been imported and colo- 
nized under more or less satisfactory conditions in America. Not all 
have successfully accommodated themselves to their new environment. 
About one-third of the total appear to have done so and to be steadily 
increasing in efficiency in accordance with their powers of multipli- 
cation and dispersion. 

It was hoped that more of them would acclimatize themselves; it 
was feared that the number might be less. On the whole, the results 



i III QIP81 MOTE IB \ i 0RE81 i.\>i I I . .*{ 

are decidedl) satisfying, and the State ol Massachusetts and the 
United States Department of Agriculture have qo cause to regrel 
having undertaken the unexpected!) formidable task of parasite 
importation. Within a territory centering a little to the uorthward 
of Boston, it may be conservative^ 9tated thai fulrj 50 per cent of 
the eggs, caterpillars, or pupa? of the gipsy moth, in the ate, 

were destroyed by imported parasites in 1912. The territory over 
which the imported insect enemies have spread is uol yel verj exten- 
sive, but ii is extending not a hi \ from year to year, and there is e\ ery 
reason to believe thai the mortality to which the gipsj moth is 
already subjected in this central portion of the infested area will 
eventually be considerably increased throughout it- whole extent, 
ae additional work will be done toward assisting in the dispersion 
of certain species, and it may be thai a ne\t attempl will be made to 
import under more sal isfactorj conditions certain others which appear 
uol to have established themselves as the result of earlier attempts. 
Otherwise the work of parasite importation may be considered as 
completed. 

THE "WILT" DISEASE OF THE QEPS1 MOTH 

More than to the parasites, more than to the perfection of the 
methods of artificial suppression, the amelioration in conditions is 
due to the "will " disease. This is a malady similar to or suggestive 
of the flacherie of the silkworm. According to recent investigations 
it is i\uv to parasitism by a bacterium which has been described under 
the name t>( Qyrococeus Hacddifex by its discoverers, Messrs. Glaser 
and Chapman, working under the direction of Dr. W. M. Wheeler, of 
the Busse) Institution. While it is uot positively proved thai this 
bacterium is the cause of the disease, there are no good grounds for 
doubting and many for believing thai it is. Confirmation is expected 
as the resull of further cooperative investigations now under way by 
the Bureau of Entomology and the Bussey Institution. 

Although we know very little of the bacterium, we know much <>f 
the malady. According to the mosl trustworthy observers it first 
appeared aboul 1903 or 1904 in certain of the worsl infested forests, 
and t>\ 1907, when the present writer lir-t became associated with the 
gipsy-moth work, it was everywhere in evidence throughout the 
infested area. It seemed slight ly to increase in the year- immediately 
following and to have reached a climax aboul 1911. At the present 
time, fortunately, there is nothing to indicate that it is at all likely 
to become much if any le>» effective in the immediate future. 

We do not yet know how the caterpillars originally become infected, 
hut once infected there is hardly room for doubting thai the organ- 
ism itself is conveyed from one generation to another through the 
Simple infection is by no mean- sufficient to cause death. On 



4 THE GIPSY MOTH AS A FOREST INSECT. 

the contrary, if conditions are otherwise favorable an infected cater- 
pillar will live, complete its transformations, and (it is believed) 
transmit the germs of the disease directly to its offspring. Under 
these circumstances generation could follow generation, and in the 
course of time the race of gipsy moths might gradually be purged of 
the disease, so that eventually only a few individuals would carry it. 

This would all be changed were the infected caterpillars to become 
weakened through any other cause. Under such circumstances an 
apparently healthy individual will sicken and die, and in a surpris- 
ingly short time the entire contents of its body will be resolved into 
a black liquid containing countless myriads of the germs of disease 
where before there were but few. Death is particularly likely to 
ensue upon the topmost twig of the tree, and the disintegrated bod}* 
of the victim, breaking of its own weight, permits the black poison 
to defile the foliage below. Another caterpillar feeding upon this 
foliage contracts the disease and, provided it also be weakened 
through any other cause, it quickly dies, and the process is repeated. 

It is very evident that the more abundant the caterpillars chance 
to be in a given forest the greater the chance that the disease will be 
thus transmitted; the more these caterpillars chance to be weakened 
through a lack of suitable food the more likely the}' are quickly to 
succumb and transmit the malady to their fellows. It thus results 
that when a forest is threatened with defoliation by infected cater- 
pillars the disease becomes epidemic and spreads with astounding 
rapidity. 

A yet more important cause for the development of the "wilt " than 
partial defoliation is to be found in unfavorable food. It used to be 
easy to rear caterpillars in the laboratory upon lettuce, for example, 
when they were free from the taint of the disease, but it is practically 
impossible to do so to-day, if American eggs are used. With foreign 
eggs, collected from a locality where the wilt is not prevalent, this 
did not prove to be the case in the course of experiments recently 
conducted at the Gipsy Moth Parasite Laboratory, which it is pro- 
posed to repeat the present year. Nor is lettuce an exception among 
foods. The same may be said of practically all herbaceous plants, 
and, fortunately, of a considerable variety of trees and shrubs. 

Herein lies the most potent cause for the less destructive character 
of the gipsy moth in recent years, according to the opinion of the 
writer — an opinion which it is expected will be abundantly confirmed 
in the course of the coming summer. And herein lies the real secret 
of the practical resistance of certain species of trees to gipsy-moth 
attack. 



Till: UIPS1 Moiii \- \ PORES! INSECT. 5 

\i; 1 11 li I M I riUZATION 01 i ill WILT " I 

h seems necessarj to emphasize the foregoing statement that the 
resistance of certain species of trees is directly due to 1 1 m ■ suscepti- 
bility of caterpillars, feeding upon the foliage of these trees, t<> death 
through the "wilt" disease, and thus incidentally i<> emphasize 1 1 1< • 
\cr\ greal importance of the disease itself. Since this disease is 
believed to result from parasitism l>\ a specific bacterium, tin- propo- 
sition of increasing its efficiency through infecting the caterpillara 
artificiallj with cultures of th< v bacterium al mice suggests itself. 

This possibility is largely precluded if equal emphasis be laid upon 
tin" real character of the disease, bo far as we are able to (let ermine it , 
either through intensive study of the organism believed to be respon- 
sible or tlirough observations upon its activities in the field. 

At the presenl time wherever caterpillars ;ire to be found, infected 
caterpillars have been found also upon everj occasion when search 
for them has been made. Furthermore, even though the infection 

were proved to he w ind-horne. as has heen contended and there is 

room for doubl regarding this mosl essentia] fact every particle of 
reliable evidence indicate- that slightly infected caterpillars remain 
reasonably healthy. The condition of the caterpillars upon the arti- 
ficially protected trees along the roadsides in localities where an epi- 
demic o( the disease prevails in the main body of the forest i-. or 
ought to be, sufficient evidence of this. Notwithstanding thai these 
caterpillars are forced to feed upon trees which have heen sprayed, 
and notwithstanding that through artificial suppression alone are 
they prevented from stripping the trees, those which escape death 
amid the various dangers by which they are artificially encompassed 
remain remarkably healthy, and with comparatively rare exceptions 
there i> an increase in cumbers of the fresh egg masses each tail over 
the number which escaped the creosote brush the preceding spring. 
Rather elaborate experiment-- have heen carried on in thepa-t 
to determine whether the disease could he practically transmitted 
through infected food, and with one not a hie exception those who have 
conducted such experiments have concluded that artificial utilization of 
the disease in this manner i- impracticable. If the writer is not mis- 
taken ir was Dr. Roland Thaxter. of Harvard University, a specialist 

of high Standing upon the vegetable parasites of insects, who was 

the lirst actually to experiment along this line, and who was the first 
to he convinced of it> futility. In 1908 Dr. Herbert Johnson, work- 
ing for the State of Massachusetts in cooperation with Harvard 
University, conducted an elaborate series of field experiments to test 
this theory, hut with no more promising results. 

Further investigation and experimentation were conducted coop- 
eratively by the State of Massachusetts and Harvard University 



6 THE GIPSY, MOTH AS A FOREST INSECT. 

under the direction of Dr. E. A. Murk and Dr. Theobald Smith. 
working more or less independently, and still the results were nega- 
tive 1 . The present writer, in connection with the work of parasite 
importation, conducted experiments of a similar nature but with the 
usual outcome. The difficulty in every case was due to the fact 
that it was impossible to secure healthy caterpillars for either the 
experiment or its check. It made no difference whether the cater- 
pillars were fed with the infected food or not, large numbers would 
die in any event, and there seemed to be no noticeable difference 
between the mortality in the experiments and in the checks. 

It remained for Mr. William Reiff, at one time a laborator}- assist- 
ant in the Bussey Institution, to claim success where others had 
failed. His experiments were, in their essential characters, like those 
of his predecessors. He fed some caterpillars upon an unfavorable 
food, and they contracted the disease and died, exactly as had resulted 
in' all other recent attempts to rear caterpillars in the laboratory from 
American eggs. When the sick and dead individuals were placed 
upon badly infested trees in the field a mortality was noticed among 
the other caterpillars in the vicinity. The fact was cheerfully ignored 
that a similar mortality might be observed in every other locality 
where the same degree of infestation prevailed, and no attempt was 
made systematically to determine exactly what happened in these 
other places. In this most important respect the experiments con- 
ducted by Mr. Reiff differed from those conducted by Dr. Johnson. 

Such a series of check observations has been made the past sea- 
son, quite incidentally, hi connection with the field-observation work 
fts conducted by Mr. A. F. Burgess of the Bureau of Entomology. 
The final results of this work for the season are not yet available, but 
the writer, who has personally visited the majority of the observation 
points, of which perhaps 20 per cent chanced to be in the immediate 
vicinity of disease plantings of the summer before, has been abso- 
lutely unable to distinguish a single point of difference between the 
treated and the untreated localities. In every badly infested locality 
complete or partial defoliation with all its attendant consequences 
resulted. The severity of the injury differed notably in different 
localities, as was to be expected, but if extreme instances were to be 
cited it would not be difficult to select localities where no disease was 
planted in which conditions at the close of the season were very 
much better than in others where plantings had been made. 

Nor is there anything unusual anywhere in the infested territory 
to differentiate conditions this fall from those prevailing a year or two 
ago. In fact it is the writer's personal impression that rather more 
pine has been seriously injured in 1912 than in 1911, and that the con- 
dition of the oak is worse than he has ever seen it before. In short, 
nothing whatever that is tangible has yet come to the attention of 



I III. i.ll'.M \ln I II VS v pokes r INSECT. , 

anyone associated with the Bureau of Entomo liich can be used 

in Biipporl of the contention that the disease ma} be rendered more 
efficient through artificial dispersion. The extensive experiments con 
ducted by the State of Massachusetts in 1912, in pursuance of its 
policy in investigate thoroughly every possible method of ridding 
its forests of the gips) moth, would appear to have resulted exactly 

as did the earlier and less elaborate aeries r lucted \<\ Dr. Johnson 

and others 

\\iii.\i CONTROl 01 I'll I " QIPS1 MOTH \lti:n\u 

A large portion of the past two years has been spenl by the writer 
abroad in studying the gips} moth in its original habitat. The 
objects of this over-sea work were several. Ii was desired more 
exactly to determine the part played by the parasites in holding 
the gips) moth in check in the European forests and to ascertain 
whether all the important species of parasites had been discovered. 
Were promising new species found, attempts wen to be made to 
ship large quantities to America for experimental colonization. 
Above all. it was hoped to learn whether the assumption upon which 
the parasite work had originally been undertaken was well grounded; 
that is, whether all the factors responsible for the natural control 
of the gipsj moth in the European forests were presenl and active 
in America, saving only the parasites. 'The results of this work are 
in part supplemented and in part confirmed by the observations of 
Mr. L. II \Vuitl,le\ . who has spenl the better part of a year abroad, 
and in part the\ are pertinent to this discussion. 

It was found that the invasion of the greater pari of Europe by 
the gipsy moth some three to se\en years ago had spent its force, 
and that, although small cumbers of the inseel might be found in 
aearl) every oak foresl visited in Italj and Germany, it was found 
abundant in verj few. It was difficult to ascertain definitely what 
had occurred to check this general invasion, bul it is certain that a 
disease similar in all its external manifestations to the American 
"wilt" (and also to the well-known and beneficent ' ' w ipfelkrank- 
heit " of the nun moth 1 had prevailed in many localities, including 
some in Kussi;i. Italy, and Germany. It seemed to have heel) epi- 
demic in all nf the localities which had been badly infested by the 
gipsy moth. 

It was also evident that following the enormous decrease in cum- 
bers brought about l>\ this epidemic there was nothing like the phe- 
nomenal increase of the straggling remainder so frequently observed 
in America. Instead, when an innocuous minimum was reached 
this desirable condition was maintained for an indefinite and some- 
times for a protracted period. It appeared highly probable and this 



8 T11K GIPS1 MOTH AS A FOBBST INSECT. 

probability was supported by definite confirmatory evidence — that 
the failure of the moth immediately to increase was due as much as 
anything to the parasites; and a wholly unexpected and phenomenal 
rate of parasitism was found to prevail in some localities. Inci- 
dentally several species of parasites until then unrecognized as impor- 
tant or promising were found and large numbers were shipped to 
America. 

The role played by the parasites, however, was obviously less 
important than had been assumed when the work of parasite impor- 
tation was inaugurated. Generally speaking, it appeared that though 
the increase of the moth was prevented or retarded through para- 
sitism, it was principally if not invariably through disease that an 
actual outbreak was checked. 

This was borne out by the circumstances associated with two local 
outbreaks in southern Italy: One in Sicily, in the extensive cork- 
oak forest of San Pietro, near Caltagirone, and the other in the 
communal forest of the town of Gioia Tauro, in Calabria. In both 
of these localities, and particularly in the latter, the parasites were 
abundant and varied. In neither were they able to prevent the 
rapid increase of the moth, much less to bring about a decrease. 
In both the defoliation of a large portion of the forest was absolutely 
complete, and in neither was the "wilt" disease operative. 

The conditions in the foresl of San Pietro were the more interesting 
and instructive because the invasion there was of no less than 12 
years' standing. At no time, according to the statements of the 
local authorities, had the entire forest been defoliated at once, but 
the invasion would sweep back and forth over it, so that the trees 
were defoliated about every second or third year. 

Here in this Sicilian forest all the amazing stories told of the gipsy 
moth upon the occasion of its historic outbreak in Maiden and Med- 
ford were abundantly substantiated. There were no streets and 
very few houses, but there were a few gardens, fields, orchards, and 
vineyards, and an abundance of wild plants, shrubby and herbaceous. 
Of them all, wild and cultivated, hardly a dozen species were immune 
from attack. There were places where at times the ground was 
black with the caterpillars as they came out of the forest, which no 
longer afforded them either food or protection, and invaded the fields 
and open spaces. Here the combination of burning sand and blazing 
sun resulted in the agonizing death of myriads, and their dead bodies 
could have been swept up by the bushel. 

Neither in 1911 nor in 1912 was a single caterpillar dying of the 
wilt observed. Parasites were abundant each year, destroying 
approximately 90 per cent, which was far from sufficient to prevent 
increase. Each year the millions which died of starvation and 



1 HE <.li'M MO I ii 18 \ l ORES i I N 3E( I . 

exposure dried up without showing traces* oj the decomposition 
in\ ariabh associated « n h i he ''will 

As a resuh , in the worst infested port ions of the f oresl there w 
be very iVw eggs, but the caterpillars would always be remarka 
health} the next Beason, and as the parasites would be attracted to 
the inure badrj infested parts of the foresl the rate of increa i i 
where would be Bimpl} astounding, 

In the Calabrian foresl the invasion had not passed beyond its 
preliminary stages in 1911, but l>\ 1912 it had reached it- mi 
mum. Here conditions were the same, pxcepl thai as the surround- 
ing fields offered better protection from the glaring sun the cater- 
pillars coming oul of the foresl lived longeranddid more damage to 
the crops. The trees in both forests were absolutel} stripped of foli 
whenever there were sufficient caterpillars and not left, as they 

commonly in America, where the disease is prevalent, with a 
sprinkling of partly eaten leaves. All kinds seemed to suffer alike. 
There were no conifers except a few cypress (related to and with 
foliage very like our white cedar which gre\s along an agave hedj 
bordering the Sicilian forest. These were stripped as bare as the oaks. 

In no locality other than these two, whether European or Amei i< 
has the writer been able to find the gipsy ninth unaccompanied by 
disease. Ii is the prevalence of such extraordinary conditions and 
their similarity to those w tiicfa prevailed in America before the devel- 
opment of the disease which serve to convince him thai the present 
improved conditions are so largely <lue to the presence of the "wilt " 
disease. 

FOREST CONDITIONS \- I FA< TOR l\ CONTROL. 

It i- obvious, even to the casual American traveler, that in a 
European countries applied forestry is developed to an unfamiliar 
extent. The forests are treated with an intelligent respecl for tl. 
requirements and a careful consideration for their continued well- 
being rarely approached in America. It maj even he -aid that th 
are among the mosl stable of European institutions. Nations h. 
risen and fallen, hut policies of foresl management adopted half a 
thousand year- ago are used as a working basis in those same fores 
to-day. 

This being the case, it logically follow- that if the gipsy moth had 
ever threatened the European forests to anything hke the ext< 
that it threatens American forests to-dai . method- of forest mana 
men! would have been evolved which, either con- or una 

sciously, would have taken it- destructive tendencies into considera- 
tion. After having studied the gipsy moth in these fore-'- both Mr. 
Worthley and the writer are much inclined to the opinion that son 
thing very like this has taken place. 



10 IKK GIPS1 MOTH AS A FOREST* INSECT. 

To a large extent the European, and particularly the German, 
forests differ radically from those within the area infested by the 
gipsy - ' moth in America. Oak, so common as a coppice growth in 
America, is relatively little grown in Germany, and when successfully 
grown*the forest bears little resemblance to the typical American 
coppice. It has been stated repeatedly that oak is not materially 
injured by the gipsy moth in Germany, even though the trees are 
occasionally stripped, because they refoliate and seem to retain a 
fail- amount of vigor. Notwithstanding this statemenl they are fre- 
quently in had condition, particularly in forests which more nearly 
approximate in character the American coppice. For example, in a 
large tract near Neusala-< )<\cv many oaks were dead or dying- Various 
causes were adduced for their death, such as attack by a species of 
Agrilus, attack by a fungous disease, attack by a leaf-roller. Tortrix 
virid'niitii, etc., and tin 1 circumstance that they had been defoliated 
several years before by the gipsy moth was not considered as respon- 
sihlc for their condition. It is significant, and suggestive of the 
existence of a parallel, that following defoliation by the gipsy moth 
•in America tin' trees frequently refoliate. hut are suhsoquently 
destroyed by a species of Agrilus, or by a fungous disease, or by 
something else than simply defoliation. 

Oak in Germany is apt to become stag-headed -that is. to die pre- 
maturely from the top; and it also happens frequently in America 
that when trees have been once or twice defoliated by the gipsy 
moth and then protected from further injury they arc affected in 
the same way. In America we have been able to determine and 
define the cause of the injury through a comparison of conditions 
outside and inside the infested territory, but in Germany, where the 
entire country may be said to be infested, such comparison is not 
so easy. 

There are numerous other points which might be brought forward 
in support of the contention that in Europe the liability of certain 
types of forests to serious injury through occasional defoliation by 
the gipsy moth has long been taken into account and that, albeit 
unconsciously, methods of forest management have been modeled 
accordingly. - Without attempting to reason out the whys and 
wherefores the pioneer foresters were content to recognize the simple 
fact that certain types of forests might not be grown under certain 
conditions or at all, and whether this were due to the character of 
the soil, or to the climate, or to the presence of an insect like the 
gipsy moth matters not. 

Similar recognition of similar drawbacks to the cultivation of cer- 
tain types of forest is probably what we must come to in America. 
It stands forth as the main result of the European observations that 
in the European forests the gipsy moth is held in check by three 



I ill i.II'm \nu II kS A ] OR! - i. 11 

urir- which did no! exist in America al the time of the earlier 
and mosl alarming in> asions 

l ["he pa 
1. The dis i 

I'll"- chars* tor of t hi fori 

The parasites are promising to become about as efficient here as 
in the European forests, and it musl not l>c forgotten that m> more 
has been claimed for them than that thej would render the gips) 
moth as innocuous here as in it-- original habitat. 

The disease, t<>". is about as efficient here as it is abroad. 

Perhaps it is not too much, then, to demand that forest conditions 
in America be made to conform a little more nearh to those of the 
countries in winch the gipsj moth is native, particularly to those of 

1 • main . 

Tli i- does no1 by any means imprj the adoption of European foresl 
methods en masse, but rather that the forests be given a little better 
attention and that provision be made f< >r the actual or inevitable 
invasion by the u r ip^.\ ninth through the elimination <>f those tn 
most likely t<> be injured and their replacement by others less 3us 
ceptible and not infrequently mine valuable. As a matter of fact 
this removal and replacement is taking place automatically in the 
territory thai has been longest infested, bul the natural process i- 
Tim often accompanied by unnecessary destruction of other trees and 
unnecessary pecuniary loss. 

RELATIVE SUSCEPTIBILITl •>! AMERICAN FORES! iREES TO QIPSY 

MOTH \ I r \i K 

As has been stated, at the time when the firsl real invasion by the 
gipsy moth in the United States was at it- height it was believed 
that scarcely any forest, shade, or fruit tree was resistant to it- 
attack. And. as has been explained, the appearance of disease, fol- 
lowing a period of uninterrupted increase of the moth, was accom- 
panied by a change in tln> respect. That this change was \rvy 
largely due to the disease is further indicated by the similarity of 
the conditions prevailing in certain Sicilian and (alahrian for 
to-day to those prevailing in Medford and Maiden in 1889 

The relative susceptibility of certain tree-, to injury and the relative 
immunity of others is therefore very largely due to, and dependei t 
upon, the presence of disease. But since i he disease is everywhere and 
bids fair to remain until possibly the gipsy moth i- freed of its taint 
through a long -erics of generations passed under ideally healthy and 
favorable surroundings, there seems t.> be no reason why the neces- 
sary dependence should not he placed in it without fear of serious 
consequences. 



12 THE GIPSY MOTH AS A FOREST INSECT. 

At the same time the writer is averse to committing himself to the 
extent of stating positively that the methods of forest management 
to be suggested further on will invariably prove successful. Ho 
wishes to emphasize the truth as he sees it. thai the relative freedom 
from injury of certain type- of forest is dependent upon the "wilt," 
and that their continued well-being will largely depend upon the 
persistence of this malady. 

lie wishes further to emphasize his belief that an increase in the 
efficiency of the parasites will add to the variety of the forests which 
may he cultivated to advantage, but that it will not result in pro- 
tection to all the types which at present are to he found in the area 
of infestation. The suggestion that experiments hi' conducted to 
determine the practicability of the "reserve-tree" method of culti- 
vating oak is made only on the assumption that the parasites will 
eventually render more efficient aid than at present and in a le>s 
limited portion of the infested territory. 

In 1908 Mr. A. II. Kirkland, in his third annual report as superin- 
tendent of work against the gipsy and brown-tail moths, first called 
attention to the apparent resistance of white pine unless associated 
with hardwood trees. A year later Mr. L. II. "Wort blew his acting 
successor, published in the next report the results of experiments 
winch showed that this was indeed the case, and that pure stands of 
pine might he protected at a very reasonable expense. 

Partly in pursuance of this idea, and partly independently, Messrs. 
1). M. Rogers and A. F. Burgess, of the Bureau of Entomology, after 
observing the activities of the moth in the field, concluded that in 
addition to pine most of the other conifers, and certain hardwood trees 
as well, might he considered as sufficiently resistant to escape serious 
injury. Ash, hickory, and maple were mentioned, and recommenda- 
tion was made jointly in Bulletin 87 of this bureau that such trees 
he planted in place of those which were destroyed. 

These recommendations, made in 1910, were succeeded by further 
observations by employees of the Bureau of Entomology and the 
Massachusetts State forester's office with the result that by the fall 
of 1911 several important additions had been made to the list. 

Exact information, however, was lacking, and because of the 
obvious need for it a series of investigations was inaugurated under 
the direction of Mr. Burgess. These were so planned as to demon- 
strate not only what happened when caterpillars were confined to 
certain soils of food indoors, hut exactly what happened following 
invasion of different types of forest out of doors. More than 250 
observation points were selected, representative of every type of pure 
and mixed forest which could he easily located within the infested 
area. In addition to pure stands of pine, oak. and birch, for example. 



I III 0IP8> Mmi ii \- \ i 0R1 BT INSECT. 1 ■ "• 

mixed Btands of oak and pine, birch and pine, oak and birch, etc., 
w ere Belectod. 

\ circle hhi feel in diameter was laid off around h central tree in 
the midst of the foresl selected, every tree included was numbered, 
and notes were made covering its species, size, general condition, and 
degree of infestation. The total number <>f egg masses t" be found 
within the circle was recorded, and their increase or decrease from 
year to year, together with the actual extent of injury resulting in 
ee of bad infestation, was to be taken as the measure of resistance 
offered bj thai particular type of forest. 

Although tin- work is n<>! yel complete, the results already Becured 
are too pertinent and valuable to be reserved until its conclusion. 

Rather for convenience than because the proposed classification 
i- altogether natural the inure common shade and foresl trees of 
New England ma\ be separated into groups in accordance with their 
wsceptibilit) to injury. 

The Bret of these groups consists of those trees upon w bich the ninth 
following its establishment norma II \ increases to the poinl of complete 
or nearly complete defoliation. After the first defoliation the moth's 
numbers may be very greatly reduced or the} may remain practically 
the same. In the one case one or more years may elapse before de- 
foliation is repeated. In the other, the forest ma\ be defoliated for 
several year- in succession. In either case the trees are likely to be 
severely injured and to die. 

This group, bo far as known, is composed exclusively of the various 
oaks, with the possible but not proved exception of the shrubby 
Bpecies These may be considered as representing the most favored 
food plants of the gipsy moth. 

The second group consists of those tree- which appear to be espe- 
cially favorable to the increase of the moth immediately following 
its establishment. Almost always, however, at about the time when 
defoliation would result were the colon] to remain healthy, it receives 
a set hack. The trees are rareh completely s< ripped, and though they 
may be from one-half to three-fourths defoliated for several successive 
years, death rarely follow-. 

The most notable representatives of this group are several of the 
tree willows and the gray birch. It i- possible thai some other trees 
will eventually he included; hut none other 30 commonly encoun- 
tered in the territory at pre-ent infested by the gipsj moth will 
Compare directly with birch or willow These, through the protec- 
tion which they afford to incipient colonic- of the gipsy moth, act 
a- incubators or breeders. 

In the third group are to he placed t ho-e trees upon which the 

gipsj moth rarely increases to the extent usual upon gray birch 01 
willow. Upon Borne it will increase until a fair degree of infestation 



14 THE GIPSY .MOTH AS A FOREST [NSECT. 

results, l)iit rarely to the point of noticeable defoliation. Upon others 
it will barely hold its own, and upon a few there wiJ] be a decrease in 
abundance following any considerable degree of accidental infesta- 
tion. These trees are what is here called resistant, and nearly all 
of them are practically that, so long as the "wilt " remains as efficient 
as it is at present. 

In this group are to be placed the pines; the spruces; in all prob- 
ability fir; hemlock, with scarcely a doubt, though it is notably more 
favorable than pine as a food plant: the junipers and cedars: doubt- 
fully larch: some, but perhaps not all, of the poplars; chestnut: 
probably beech: yellow birch, black birch, and probably paper birch: 
apparently all the species of hickory; butternut: sycamore: Ameri- 
can elm, and probably the other species of elm: apparently hack- 
berry; sassafras: catalpa; the various species of ash: black locust 
and honey locust: black cherry and bird cherry: probably mountain 
ash: all the indigenous and probably the European species of maple, 
although the Norway maple is more liable to attack than others: 
boxelder; tupelo; horse-chestnut; ailanthus; tulip tree, and undoubt- 
edly many other of the less commonly planted shade and ornamental 
trees. 

A few of the more common trees have not yet been definitely 
placed, notably basswood or linden, ironwood. and hop hornbeam. 

RELATIVE SUSCEPTIBILITY OF MIXED FORESTS. 

It would thus appear that in a territory in which both disease and 
parasites, or disease alone, is prevalent the gipsy moth becomes 
peculiarly an enemy of oak, and this is true in so far as pure stands 
of trees are concerned, or of isolated trees. It is not so true of mixed 
stands, however, as was pointed out by Mr. Kirkland and by Mr. 
Worthley in respect to pine mingled with hardwood, and these mixed 
stands may generally be considered just a little more resistant than 
would be a pure stand of the least resistant tree and considerably 
less resistant than a pure stand of the most resistant tree which goes 
to make up any considerable portion of the mixture. Thus a pine 
and oak forest is slightly less liable to injury than a pure stand of 
oak and much more liable than a pure stand of pine. The same 
might be said of an oak and hickory mixture or one of oak and 
chestnut. The reason is that the caterpillars, increasing uninterrupt- 
edly upon oak. will finally be forced to leave it and will strip other 
trees upon which they would not increase to anything like a similar 
extent if they were forced to feed upon them for generation after 
generation. They do not always do this, it is true, but they do it 
very often, particularly when the oak is abundant and scattered 
evenly throughout the forest. 



I ill. GDPB1 HOTS 18 \ i 0BE8T tv-M i . 1 B 

Since hemlock i- better liked Mian pine l>\ tin- caterpillars, a mixed 
growth of oak ami hemlock is much more likely in he destroyed com- 
pletely than :i mixed growth «>t' oak ami pine: ami since graj birch 
i- ii< » i -i. favorable a food plant as oak, a birch and pine mixture ie 

not nearly ><> likely In Buffer a- a mixed -land of nak ami pine. 

Apparent!} no fear oeed he felt as to the safety of anj mixture 
whatever of which all the component parts could he considered as 
resistant it" they were Btanding in pure growth. As •<> the greater 
resistance of an oak tree when Btanding Burrounded l>> chestnut ami 
hickory, a- compared with another of the same Bpecies and vigor 
surrounded by oak. there is room for Further investigation. It can 
only he -aid that the protection thus afforded i<> oak through being 
associated with other tree- i- not particularly Btriking. 

RELATION or UNDERBRUSB TO THE FOREST. 

li i- interesting, in connection with the relative resistance of forest 

trees, to note that the aprouts and, t>> a Ie-- extent, the seedlingB, are 

not bo liable to injury a- are Larger tree- of the same Bpecies. This 
i- tine even of pine, mile— the writer ha- misinterpreted his field 
observations. 

Il i- al-o true thai for the nio-t part tin- common species ol -liruh- 
to he found growing a- u.nderhru-h in a forest air unfavorable BS 
food for tin- gipsy moth that i- to -ay. they ma\ he classed with 

the resistant species of trees. It i- certainly logical and. to that 
extent, reasonable to suppose that underbrush will he found to play 
quite an important part in the protection of the forest. Caterpillars 
falling from tree- in a pure -land of oak devoid of underbrush will 
find their way back to oak and be little tin- worse for the adventure. 
Caterpillars falling in a similar manner in a forest full of underbrush 
will not find their way hack bo readily, and the eating of strange 
food for h time will render them Ie— resistant to disease, more likely 
to die. and therein more likely to transmit the germs of disease to 
their fellows. 

It is a subject well worthy of further study ami experimentation 
and one to which it is hoped to devote considerable attention in the 
course of the coming year, the mote -o since it ha- special bearing 
upon the suggestion that oak might possibly he protected by adopt- 
ing the method ot cutting so a- to leave reserve trees. 

BE] \TIY1. RESISTANCE OF DIFFERENT TREES OF tut SAME SPECIES. 

It would appear from numerou- observations that certain indi- 
vidual trees (of red oak. for example) are much better able to with- 
stand the attack ot the irip^y moth than others of the same species 
growing in the same wood lot. These tree-, although they are sub- 
jected to the same degree of defoliation, will live when all around 



16 THE GIPSY MOTH AS A FOREST INSECT. 

them die and even take on new life and vigor through coming into a 
larger share of light and space. It is yet to be determined whether 

these trees are more resistant or whether, through standing in some 
favored pockets of richer soil, they simply survive through the pos- 
session of a more vigorous constitution. 

It is undoubtedly true that vigorous, rapidly growing trees are 
more resistant than other trees of the same species less vigorous 
and less rapidly growing. Thus oak trees around the border of an 
infested wood lot, with more room to expand their roots and branches, 
not infrequently live when all or nearly all the more crowded indi- 
viduals in the depths of the wood lot die. It is also true that isolated 
trees withstand a greater degree of defoliation than those in dense 
growth. 

Advantage seems to have been taken of this principle in the method 
of growing oak formerly in vogue in Germany, for, if the writer is 
correctly informed, the finest and largest oaks in the Empire are 
grown more or less isolated and parked. When the relative immu- 
nity of young sprouts is also considered, a very logical reason is 
suggested for the fact that the method of leaving reserve trees, 
scattered over a territory devoted principally to the growth of sprouts 
or coppice, finds so much favor in certain parts of Europe. The-e 
reserve trees are left practically isolated in the forests which have 
come to the writer's personal attention, and the sprouts, for which 
there is a read} 7 market, unfortunately lacking in America, are cut 
at quite frequent intervals. 

SUGGESTIONS FOR THE MANAGEMENT OF FOREST LANDS WITH REFER- 
ENCE TO THE GIPSY MOTH. 

It is by no means to be understood that because of the gipsy moth 
all the oak in this country is foredoomed to destruction, but atten- 
tion must be called to what is undoubtedly the truth, that unless 
parasitism or disease, or something else not at present recognized as a 
variable factor in the natural control of this pest, develops to an 
extent unknown in either America or Europe, pure and mixed stands 
of oak will be seriously injured. Not only is the oak itself liable to 
injury, but also other trees mixed with it. 

It is yet too soon to state definitely whether, in localities where 
there is no oak, other trees may not act as breeders of the moths to 
an equally disastrous extent. For instance, the paper birch in the 
north woods may so foster it as to bring about the defoliation of 
adjacent spruce. The results of field-observation work in occasional 
bits of forest in the infested territory when 1 spruce and birch occur 
are not in themselves sufficient to settle the question, because condi- 
tions so far from the large body of boreal forest can not be considered 



i in-: i.ll-s\ Mm ii \> \ i (mm ST INBBOT. 17 

as rralh typical. s.> far as they go, however, they indicate thai the 
gipsy moth is not to l)<" feared outside the range of the oak dr in 
forests inside that range provided the oak, and possibly one or two 
other species of unimportant trees, be eliminated. 

There are, therefore, two phases of the complex problem of gipsy- 
moth control in forests which must be considered. Firet, how best 

to eliminate the oak and -ecure it- replacement by other ami, if 

possible, more valuable trees; and, second, how besl to proteel the 

oak from aerious injury in localities where little else can he grown to 

adi antage. 

In a large portion of the area at present infested by the lt i j > - > 
moth the solution is almost absurdly simple. This is the natural home 
of the white pine, one of the most valuable timber trees to be found 
in the whole Temperate Zone. In a way the oak is an interloper. 

Over a large part of New England the white pine was once preeminent . 

and it would become so again were the country to be deserted by 

civilized man. The pine reproduces freely, if given half a chance, 
hut there are thousands of acres in the aggregate in which a natural 
reproduction of pine is being retarded, destroyed even, through the 
mare circumstance that the oak chanced to secure a running start, 
by sprouting, when the Land was last cut over. The German forester 
who would permit such conditions to prevail would be considered 
hopelessly, even criminally insane. Under such circumstances oak 
is to be considered a- a weed, and the advent of the L'ip^y moth as a 
blessing when, a-- sometime-- happens, it takes the oak and leaves the 
pine. If it would always do jusl that and nothing more it- progress 

mighl be watched with a certain degree of complacency. But it does 

not always stop at that and, what i> worse, injudicious cutting not 
infrequently results in greater damage than would be done by the 

gipsy mot h it-elf. The larger pines are apt to be cut or broken down, 
and the -mailer one-, unable to compete with the rapidly growing 
oak sprout-, are quickly in no better condition than before. 

The natural program, therefore, in every pine and oak mixture, 
i- so to eliminate the oak a- to afford the pine a better opportunity 
to take possession of the ground. How thi- may he-t be accom- 
plished depends entirely upon the individual characteristics of any 
particular wood lot. And. furthermore, it i- strictly a problem in 
applied forestry and one for the forester, not for the entomologist, 
to solve. 

In a great many localities where the white pine doe- not grow 
naturally, or in winch it has been destroyed through injudicious cut- 
ting and extensive forest tire-, there is to be found a stand of oak 
mingled with other hardwoods. In these forests tin- solution is not 
reached quite so easily or -o satisfactorily. Chestnut (saving only 



18 THE GIPSY MOTH AS A FOREST INSECT. 

for the possible injury by the blight), hickory, paper birch, sugar 
maple, and ash all find conditions greatly to their liking in one or 
another part of New England, and all are desirable substitutes for 
oak. There are certainly some, and probably numerous localities in 
which, through nothing more than the exercise of a little care and 
intelligent management, the oak may be removed and its natural 
replacement by these other hardwoods may be secured with a mini- 
mum of expense. 

It is in the pure stands of oak, or in those of oak mingled with much 
less valuable trees, all too frequently to be found throughout the 
infested area, that the problem becomes acute. Here the land must 
be allowed practically to go to waste, or planting must be resorted to, 
or else some attempt must be made to maintain a growth of oak. 
The most simple method would seem to be the leaving of reserve 
trees. The chances at present are that this may not work very well, 
but unless the relief which we have every reason to expect through the 
further development of the parasites be denied us, there is a chance 
that in the near future it will prove to be a fairly satisfactory, cheap, 
and eventually remunerative alternative to permitting the forests to 
be entirely destroyed. It must not be forgotten that when the oaks 
are left to die from the gipsy-moth attack they very rarely sprout, 
and a pure oak stand is apt to degenerate into a thicket of gray birch 
or something even less intrinsically valuable. If the oak be thinned 
in advance of the gipsy-moth invasion the sprouts will be resistant 
for a period of years at least, perhaps until the parasites become so 
efficient that they wall protect the stand of reserve trees. It is even 
possible, if the thinning be done far enough in advance of the inva- 
sion, that the reserve trees will have increased sufficiently in vigor 
to resist the attack of the moth until the parasites shall have multi- 
plied sufficiently to hold it in check. 

All these suggestions are to be treated as such and as nothing 
more. The author is no forester and can not pretend to recommend, 
but only to suggest. He has had the advantage, however, of four 
years' study of the insects injurious to American forests under the 
direction of the foremost forest entomologist in America, and through 
subsequent study of the gipsy moth in America and abroad has 
reached certain pretty definite conclusions. The gipsy moth is 
distinctly a menace to our forests, but it is really no more to be 
feared than any one of several forest insects native to tins country. 
If the situation be rightly viewed, and a serious attempt be made 
to cope with it, it is certain that the results will redound not only 
to the benefit of the forests in general but to that of the country 
at large. 



I ill QIP81 \i"i ii IB A 1 OR] BT INSECT, I '» 

1111: i\i\iim\m 1:1 .i,H ii.-i MIA i - 01 Mil Mil \TH>\ 

The immediate requirements of the situation are thai the work 
be conducted in a hearts spirit of cooperation among all concerned. 
For tin' purpose of solving t li<' problems associated with the elimina- 
tion of oak and its replacement by other and, if possible, more 
valuable trees the Bureau of Entomology has allied itself with the 
United Stales Forest Service and hopes also to continue in hearty 
accord with the various State foresters most immediately concerned. 
The further investigation of the "wilt" disease will l>e conducted 
l>\ the Bureau of Entomology and the Bussej Institution working 
together. Additional information concerning the relative resist- 
ance of various t ices standing singly, or in pure or mixed growth, 
will be compiled by the Bureau of Entomology, which will also 
concern itself to discover exactly whal progress is being made by 
thi> imported parasites and predatory enemies. These lines of 
investigation, observation, and research are all being directed with 
one single end in view, hov besl to protecl the forest. 

Of greal importance, also, is the work intended to restrict the 
spread of the gipsy moth beyond the boundaries of New England. 
A certain amount of natural spread can not l>e prevented, especially 
through uiml. but it U hoped entirely to eliminate the danger of 
an immensely more rapid spread through artificial channels, upon 
Qursery Btock, forest products, etc., shipped from infested to oonin- 
fested territory. It is also hoped considerably to retard the slow 
and inevitable natural dispersion, even though it is impossible 
entirely to prevent it. 

The natural progression of tlie moth to the westward, which is 
the most to be feared and also the easiest to retard, will be more 
effectively controlled if the woodland colonies along the western 
frontier, and for that matter over the whole infested area, are sup- 
pressed. It is from these that the wind spread largely comes about. 
The longer they can be kept down, the less the likelihood of their 

becoming a source of infestation to the country beyond. 

Especially along the frontier the colonic-, .ire at first feu ami 
scattered, and while everything within reason will be done toward 

their suppression, there is believed to be justification for asking the 

active cooperation of foresl owner-- directly interested. If through 
a modification in their methods of h andling their property they can 

save themselves from what would now appeal- in many in-~t.ii 
to be certain loss; if they can at the same time put their fort 
in such condition a- not only to protecl them in the future but also 
to render them more intrinsically valuable; and. finally, if they 
can render a not inconsiderable public service through helping to 



20 THE GIPSY MOTH AS A FOREST INSECT. 

retard the progress of the moth into territory not as yet infested, 
the request for such cooperation would appear to be well justified. 

This project, to bring good out of the evil that has resulted through 
the establishment of the gipsy moth, by combating it through 
methods which would make American forests more valuable than 
they have ever been before, is no mere vision. Evidence enough 
of its practicability in the case of the mixed stands of oak and pine 
already referred to is easily found and sufficiently convincing. 
That other types of natural forests may be handled so as to make 
the outcome advantageous to the forest owner as well as to the 
whole country, it is only reasonable to expect. 

The Forest Service and the Bureau of Entomology will attempt 
to do their share of the work, through cooperative study of the 
technical aspects of the problem. Experimental and demonstrative 
work must precede definite recommendation, and in this the aid and 
assistance of the forest owners themselves must be secured. If a 
spirit of hearty cooperation can be established and maintained it 
would seem as though the problem would all but solve itself. 

Approved: 

James Wilson, 

Secretary of Agriculture. 
Washington, D. C, November J, 1912. 



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